Among purple martins, experienced males
mate with their own partners early so
they may devote themselves to
cuckolding neophite males
later. By singing distinctive
tunes, elders attract younger
males, who attract more
females. Elder males
then sire extra offspring
with the debutante females,
but leave the job of rearing the chicks to the younger males.

Reported by Natalie Angier in the New York Times